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Transcript of Telescopes
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Telescopes
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• Basic function of a telescope: extend human vision– Collect light from celestial object
– Focus light to create image or spectrum of the object
– Use larger aperture than the human eye
– Expose for longer than the human eye
– Achieve better resolution than human eye
– Observe at wavelengths the eye is not sensitive to (i.e. beyond 400 – 700 nm)
– Examine spectral information in detail
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Light Hitting a Telescope Mirror
huge mirror near a star
small mirror far from 2 stars
In the second case (reality), light rays from any single point of light are essentially parallel. But the parallel rays from the second star come in at a different angle. 3
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If the mirror is a particular shape, a paraboloid, light rays from a distant source, parallel to the "mirror axis" all meet at one point, the focus.
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Image Formation
"focal plane"
Light rays from a distant, extended source are all focused in the same plane, the "focal plane" creating an image of the source.
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Optical telescopes Kinds of optical telescopes:
1) Refractor – uses a lens that light passes through, to concentrate light. Galileo’s telescope was a refractor.
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<-- object (point of light) image at focus
• Lens can only be supported around edge
• Some light absorbed in glass (especially UV, infrared)
• Air bubbles and imperfections affect image quality
• "Chromatic aberration"7
Problems with Refracting Telescopes
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Chromatic Aberration
Lens - different colors focus at different places.
white light
Mirror - reflection angle doesn't depend on color.
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blue focus red focus
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Largest Refracting Telescope Built
Yerkes 40-inch (about 1 m).
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Solution:
2) Reflecting telescope
use concave mirror (shape is ideally parabolic), not lens, to focus light. Newton built first one. Big, modern research telescopes are reflectors.
10Gemini South 8-m reflector.
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Reflector advantages• Mirrors can be large, because they can be supported from
behind.
• No chromatic aberration
• Less light lost and fewer image quality problems
Largest single mirror built: 8.4 m diameter for the Large Binocular Telescope
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• There are 10 m telescopes, but in segments
Keck 10-m telescope12
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focus options
or Nasmythfocus
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Nasmyth focus platforms
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Image of Andromeda galaxy with optical telescope.
Image with telescope of twice the diameter, same exposure time.
Characteristics of telescopes• Light gathering power: area, or D2 Main reason for building
large telescopes!
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Characteristics of telescopes, cont.
• Magnification: angular diameter as seen through telescope/angular diameter on sky– Typical magnifications 10 to 100
• Field of View: how much of sky can you see at once? Typically many arcminutes – few degrees.
• Resolution: The ability to distinguish two objects very close together. Angular resolution:
θ = 2.5 x 105 /D where θ is angular resolution of telescope in arcsec, is wavelength
of light, D is diameter of telescope objective, in same distance units.
• Example, for D=2.5 m, λ=500 nm, θ = 0.05”17
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Two light sources with angular separation larger thanangular resolution vs. equal to angular resolution
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But, “seeing” limits resolution for ground-based optical telescopes
* Air density varies => bends light. No longer parallel
Parallel rays enter atmosphere
CCD
No blurring case. Rays brought to same focus.
* Sharp image on CCD.
Blurring. Rays not parallel. Can't be brought into focus.
Blurred image.
resolution limited to about 1”19
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fuzziness you would see with your eye.
detail you can see with a telescopeon ground.
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Example: the Moon observed with a 2.5 m telescope
1" => 2 km
0.05" => 100 m
Ground-based telescope image, 1” resolution
Hubble SpaceTelescope image,0.05” resolution 21
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DetectorsQuantum Efficiency = how much light they respond to:
– Eye 2%
– Photographic emulsions 1-4%
– CCD (Charge coupled device) 80%• Can be used to obtain images or spectra
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CCDs also provide data directly in digital form – easier to process.
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Photographic film CCD
Same telescope, same exposure time!23
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Spectrographs: light spread out by wavelength, using prism or “diffraction grating”
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Some future optical telescopes
Large Synoptic Survey Telescope (LSST): 8-m telescopewith large field of view (3.5°). Will survey entire skyrepeatedly. Site in Chile. First light 2019.
Thirty Meter Telescope (TMT):
segmented design, like Keck.First light 2022. 25
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Radio Telescopes
Large metal dish acts as a mirror for radio waves. Radio receiver at prime focus.
Surface accuracy not so important, so easy to make large one (surface shouldn’t have irregularities that are larger than 1/16 ).
But angular resolution is poor. Remember: θ = 2.5 x 105 /D
D larger than optical case, but much larger (cm's to m's), e.g. for = 1 cm, diameter = 100 m, resolution = 20".
Andromeda radiomap with Effelsberg telescope
Effelsberg 100-m (Germany)
Andromeda galaxy –optical
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Parkes 64-m (Australia)
Green Bank 100-m telescope (WV) Arecibo 300-m telescope (Puerto Rico)
Jodrell Bank 76-m (England)
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• So how can we get better resolution?
• Interferometers – e.g., VLA
Use interference of radio waves to mimic the resolution of a telescope whose diameter is equal to the separation of the dishes
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InterferometryA technique to get improved angular resolution using an array of telescopes. Most common in radio, but also limited optical interferometry.
D
Consider two dishes with separation D vs. one dish of diameter D. By interfering the radio waves from the two dishes, the achieved angular resolution is the same as the large dish. 29
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Example: wavelength = 1 cm, separation = 2 km, resolution = 1"
Very Large Array (NM). Maximum separation 30 km (only about 1km in thisconfiguration).
Very Long Baseline Array. Maximum separation 1000's of km
VLA and optical image of Centaurus A
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Atacama Large Millimeter Array
• 18,000 ft elevation plateau in Chile• USA/Europe/Japan collaboration• Started observing in 2011 with a few dishes• 66 dishes eventually
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• UNM is building its own array for =3-10m: the Long Wavelength Array (LWA)
• Far larger than the VLA, to give same resolution. “Stations” of 256 antennas, to be spread across NM
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• Square Kilometer Array, currently being designed, will be 50 times collecting area of VLA, with baselines to 1000’s of km 33
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Optical-to-mm-wave Telescope Sites
• Site requirements– Dark skies (avoid light pollution)
– Clear, dry skies
– Good “seeing”, stable atmosphere
• High, dry mountain peaks are ideal observatory sites, for optical to mm waves.
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USA at night
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Mauna KeaObservatory,Hawaii
Kitt PeakNationalObservatory,Arizona 36
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Radio Telescope Sites
• Away from radio interference is most important. Radio astronomy can be done in cloudy weather, day or night.
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Telescopes in spacePros – above the atmospheric opacity so can work at
impossible from ground, above turbulence, weather, lights on Earth
Cons – expensive! Repairs difficult or impossible.
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Spitzer Space Telescope - infrared
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Shorter infrared wavelengths allow you to see through dust. Dust is good at blocking visible light but infrared gets through better.
Trifid nebula in visible light Trifid nebula with Spitzer
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Longer infrared wavelengths allow you to see radiation from warm dust in interstellar gas
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FERMI – Gamma Ray Telescope
Gamma rays are the most energeticphotons, tracing high-energy events inUniverse such as “Gamma-ray Bursters”.
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Hubble Space Telescope and its successor-to-be: the James WebbSpace Telescope
Advantage of space for optical astronomy: get above blurring atmosphere – much sharper images.
Center of M51: HST (left; 0.05” resolution) vs.ground-based (right; 1” resolution)
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The JWST
Diameter 6.5 meters (vs. HST 2.5 meters) – much higher resolution and sensitivity. Will also observe infrared, whereas Hubble is best at visible light. Expected launch 2018.
Mock-up of JWST
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